[083] Noninvasive Loop Gains from Output Impedance Measurements

Introduction

It is always desirable to simplify the measurement process of power supplies, and characterize them without overly invasive testing. In this article, Dr. Ridley shows how this can work for loop gains with limited success, and shows the pitfalls of trying to measure loops noninvasively.

Power Supply Output Impedance Measurements

For decades now, engineers have followed Middlebrook’s techniques for breaking and measuring feedback loop gains while keeping the feedback intact for regulation. However, in some cases, chip designers no longer make loop feedback points available, or board layouts prevent proper direct measurement. Or,you may just have a black box power supply with no access to interior nodes of the circuit.

It is well known (again from Middlebrook) that you can get the loop gain indirectly by looking at the output impedance of the power supply. Figure 1 shows the conceptual idea behind measuring output impedance.

Fig. 1: Output impedance is measured by driving a current into the output terminals of a power supply